The invention relates to a method and to an apparatus for measuring scattered light on an optical system. The invention further relates to a projection exposure tool for microlithography with this type of apparatus and to a mask for measuring scattered light on an optical system. Further the invention relates to a method of operating a projection exposure tool for microlithography.
With systems for microlithography scattered light measurement is a very important measuring method in order to be able to qualify the associated optical system. Therefore, for the qualification of an optical system for microlithography, such as for example a projection objective, it is no longer sufficient to only measure the low aberrations or imaging errors, and in fact for an accurate forecast of the lithographic properties of the optical system knowledge of the scattered light portion of the light radiation conveyed through the optical system is of crucial significance. Generally, with optical systems for lithography the scattered light portion is determined with a mask which has an opaque square structure in a translucent environment. The mask is exposed in a number of steps with an increasing dose until the photoresist is developed on a wafer in the image plane of the optical system at the location of the opaque structure. The information regarding the scattered light portion in the optical system obtained with this method is however often not sufficiently meaningful, in particular for microlithography.